The alternating-current power grid developed in the late nineteenth century with features such as centralized unidirectional electric power transmission and demand-driven control. In the twentieth century, utilities inter-tied small local grids to form larger and larger power grids, which lent to efficiencies of scale. However, near the end of the twentieth century, the economies of scale of power production were limited by difficulties in propagating supply and demand price signals through the system, environmental concerns about power production, and an increased dependence on limited fossil fuel resources.